Formation of carbon nanotube-containing devices

ABSTRACT

A method of fabricating a carbon nanotube based device, including forming a trench having a bottom surface and sidewalls on a substrate, selectively depositing a bi-functional compound having two reactive moieties in the trench, wherein a first of the two reactive moieties selectively binds to the bottom surface, converting a second of the two reactive moieties to a diazonium salt; and reacting the diazonium salt with a dispersion of carbon nanotubes to form a carbon nanotube layer bound to the bottom surface of the trench.

BACKGROUND Technical Field

The present invention relates to a semiconductor structure, and moreparticularly to fabrication of a transistor structure self-assembledcarbon nanotube (CNT) technology.

Description of the Related Art

Carbon nanotubes (CNT) possess properties that make them an optionalmaterial for use in various electronic devices, such as field effecttransistors (FETs). The CNT may be used in place of silicon for achannel material, however, use of CNTs involves forming a useful layerof the material in a controlled and/or predictable manner. Althoughdifferent attempts have been made to prepare such CNT layers, furtherimprovements would be beneficial.

SUMMARY

A method of fabricating a carbon nanotube based device, includingforming a trench having a bottom surface and sidewalls on a substrate,selectively depositing a bi-functional compound having two reactivemoieties in the trench, wherein a first of the two reactive moietiesselectively binds to the bottom surface, converting a second of the tworeactive moieties to a diazonium salt; and reacting the diazonium saltwith a dispersion of carbon nanotubes to form a carbon nanotube layerbound to the bottom surface of the trench.

A method of fabricating a carbon nanotube based device, includingforming a metal oxide layer on a substrate, wherein the metal oxidelayer has an exposed surface, forming one or more non-metal oxideislands on the metal oxide layer, wherein portions of an underlyingmetal oxide layer surface remain exposed, contacting at least theexposed portions of the underlying metal oxide layer surface with asolution of a bi-functional compound including an aniline moiety and ahydroxamic acid moiety to form a monolayer on the exposed portions ofthe underlying metal oxide layer surface, reacting the aniline moiety toform a functionalized bi-functional compound, and contacting thefunctionalized bi-functional compound with a carbon nanotube dispersionto form carbon nanotubes bound to the functionalized bi-functionalcompound.

A carbon nanotube based device, including a substrate, a metal oxidelayer on the substrate, a plurality of non-metal oxide islands on themetal oxide layer forming a trench between the non-metal oxide islands,a monolayer of a bifunctional compound in the trench, wherein thebifunctional compound is selectively chemically adsorbed onto the metaloxide layer, and a plurality of carbon nanotubes chemically bonded tothe monolayer of the bifunctional compound.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional diagram of a CNT-based device in accordancewith an exemplary embodiment;

FIG. 2 is a cross-sectional view of a CNT-based device shown in FIG. 1with a metal oxide layer in accordance with an exemplary embodiment;

FIG. 3 is a cross-sectional view of a CNT-based device shown in FIG. 2with a patterned oxide layer in accordance with an exemplary embodiment;

FIG. 4 is a cross-sectional view of a CNT-based device shown in FIG. 3with a bi-functional compound on the exposed portions of the underlyingmetal oxide layer surface in accordance with an exemplary embodiment;

FIG. 5 is a cross-sectional view of a CNT-based device shown in FIG. 4with a functionalized bi-functional compound on the exposed portions ofthe underlying metal oxide layer surface in accordance with an exemplaryembodiment; and

FIG. 6 is a cross-sectional view of a CNT-based device with carbonnanotubes (CNTs) bound to the functionalized bi-functional compound, asshown in FIG. 5 in accordance with an exemplary embodiment.

DETAILED DESCRIPTION

Principles and embodiments of the present disclosure relate generally toa semiconductor structure including carbon nanotubes, where the CNTsform a layer of the structure. The structure may be part of asemiconductor device or form a complete semiconductor device. In variousembodiments, the semiconductor device may be a transistor, or moreparticularly a field effect transistor (FET), where the carbon nanotubesreplace silicon as the channel material. It is also contemplated thattwo or more semiconductor devices may also be combined to form a logicdevice, for example, a CMOS device or a gate device.

Principles and embodiments also relate to a field effect transistorhaving a channel layer formed by self-assembled carbon nanotubes, wherethe carbon nanotubes deposit as a monolayer in a defined recess in thedevice surface.

It should be understood that reference to carbon nanotubes is fordescriptive purposes only and intended to encompass the differentvarieties of 1-D and 2-D nanostructures, including but not limited tosingle-walled nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs),multi-walled nanotubes (MWNTs), and chemically modified nanotubes.

In various embodiments, the carbon nanotubes are not reacted with othersurface active agents (surfactants) to control or modify placement ofthe CNTs on a surface. In various embodiments, the carbon nanotubes arenot associated with one or more compounds having at least one functionalgroup prior to placement on a surface. In various embodiments, theplacement of the carbon nanotubes onto prescribed locations on thesurface of a substrate may be accomplished without chemically (e.g.,functionalizing the CNTs with a moiety) or physically (e.g., associatinga moiety with the CNT by physical interaction(s), for example,entanglement, polymer wrapping, or physisorption).

In one or more embodiments, carbon nanotubes suspended in organicsolvent(s) may be selectively placed onto pre-patterned surfaces usingdiazonium chemistry. A surface may be pre-patterned using abi-functional compound having an aniline moiety and a hydroxamic acidmoiety. A surface-reactive agent may be end-functionalized to include ananiline moiety on one end and a hydroxamic acid moiety on the other endof the molecule. (See Formula I).R′(HO)N(O)C—R¹—C₆H₄—N═N(Cl)  (I)

In one or more embodiments, R¹ may be an aliphatic (—CH₂—) chain with alength in the range of C1 to C18, or C3 to C12, a cyclic aliphatichaving a ring size in the range of C5 to C7, or a combination thereof.In various embodiments, the bi-functional compound may have thefollowing structure, (see Formula II):

In various embodiments, a monolayer may be formed by the bifunctionalcompound including a hydroxamic acid group at one end of an aliphaticchain and an aniline functional group on the opposite end of thealiphatic chain, where the hydroxamic acid group binds to the metaloxide surface. The bi-functional compound may form the monolayer on asurface by having the hydroxamic acid moiety selectively bind to a metaloxide surface region.

In various embodiments, an aniline moiety may be converted to adiazonium salt with the addition of amyl nitrite, and the diazonium saltreacted with a carbon nanotube to chemically bind the carbon nanotube tothe metal oxide surface region. (See Formula III).

A monolayer may be formed by the bifunctional compound including ahydroxamic acid group at one end of an aliphatic chain and the diazoniumsalt on the opposite end of the aliphatic chain, where the hydroxamicacid group binds to the metal oxide surface carbon nanotube and thediazonium salt binds to the carbon nanotubes. (See Formula IV).HfO|—ONR′(O)C—R¹—C₆H₄—N═N—[CNT]  (IV)

In various embodiments, the hydroxamic acid moiety selectively binds tothe surface oxygens through the carbonyl group and the hydroxyl group toform a seven-membered cyclic structure. The bi-functional compoundselectively binds to the oxygens of the metal oxide (e.g., HfO₂, Al₂O₃,etc.) over the non-metal oxides (e.g., SiO₂). Without being bound bytheory, it is believed that the basic surface of the metal oxide, whichcan have an isoelectric point of about 6 to about 9, reacts with theacidic hydroxamic acid moiety to form the cyclic structure through twobonds.

In one or more embodiments, the end of the bi-functional compound notbound to the metal oxide surface may be reacted to form a diazonium saltin situ by reaction with a nitrite.

In various embodiments, a plurality of CNTs may bond to thebi-functional compound monolayer on the metal oxide surface. Each CNTmay bond to one or more bi-functional compounds.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

While exemplary embodiments have been shown for a particular device, itshould be understood that a plurality of such devices may be arrangedand/or fabricated on a substrate to form integrated devices that may beintegrated onto a substrate, for example through very large scaleintegration to produce complex devices such a central processing units(CPUs) and application specific integrated circuits (ASICs).

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, which is a cross-sectionaldiagram of a CNT-based device in accordance with an exemplaryembodiment.

In one or more embodiments, a CNT-based device 100 includes a substrate110. A substrate 110 may be a metal, semiconductor, or an insulator. Thesubstrate 110 may be crystalline, semi-crystalline, microcrystaline, oramorphous. A metal substrate may be, for example, aluminum, hafnium,copper, titanium, as well as other metals that form a thin layer ofoxide on the surface that can bind with hydroxamic acids. Asemiconductor substrate may be essentially (i.e., except forcontaminants) a single element (e.g., silicon), primarily (i.e., withdoping) of a single element, for example, silicon (Si) or germanium(Ge), or the substrate may be a compound, for example, GaAs, SiC, orSiGe. An insulator substrate may be aluminum oxide (Al₂O₃), siliconoxide (SiO₂), etc. The substrate may also have multiple material layers,for example, a semiconductor-on-insulator substrate (SeOI), asilicon-on-insulator substrate (SOI), germanium-on-insulator substrate(GeOI), or silicon-germanium-on-insulator substrate (SGOI). Thesubstrate may also have other layers forming the substrate, includinghigh-k oxides and/or nitrides. In one or more embodiments, the substrate100 may be a silicon wafer. In an embodiment, the substrate is a singlecrystal silicon wafer.

FIG. 2 is a cross-sectional view of a CNT-based device shown in FIG. 1with a metal oxide layer in accordance with an exemplary embodiment.

In various embodiments, a metal oxide layer 120 may be formed on thesurface 115 of the substrate 110. The metal oxide layer 120 may have asurface 115 that can form a chemical bond with a hydroxamic acid(—C(O)NR′OH) moiety. In various embodiments, the metal oxide layer 120may be a high-k oxide. The metal oxide layer 120 may be selected fromthe group consisting of HfO₂, ZrO₂, HfSiO, HfSiON, TiO₂, Y₂O₃, Al₂O₃,Ta₂O₅, InBaTiO₃, BaSrTiO₃ (BST), SrTiO₃, and combinations thereof. Inone or more embodiments, the metal oxide layer 120 may be HfO₂, ZrO₂,TiO₂, or Ta₂O₅.

In embodiments in which the substrate is a metal, the substrate may havean oxide layer suitable for binding to the hydroxamic acid (—C(O)NR′OH)moiety, or be treated to form an oxide layer suitable for binding to thehydroxamic acid (—C(O)NR′OH) moiety.

In various embodiments, the metal oxide layer 120 may have a thicknessin the range of about 1 nm to about 100 nm, or in the range of about 5nm to about 20 nm, although other thicknesses are contemplated, and ametal substrate having a much greater thickness may be used.

The metal oxide layer 120 may be formed on the exposed surface 115 ofthe substrate 110, where the metal oxide layer 120 may be formed by oneor more deposition processes (e.g., physical vapor deposition, chemicalvapor deposition, atomic layer deposition), spin coating, orcombinations thereof. The metal oxide layer 120 and exposed surface 115of substrate 110 may form an interface, where the metal oxide layer 120and exposed surface 115 are in contact.

FIG. 3 is a cross-sectional view of a CNT-based device shown in FIG. 2with a patterned oxide layer in accordance with an exemplary embodiment.

In one or more embodiments, a patterned non-metal oxide layer may beformed on the metal oxide layer 120 to create non-metal oxide islands130. In various embodiments, the non-metal oxide layer and non-metaloxide islands may be silicon dioxide (SiO₂), silicon oxynitride (SiON),germanium oxide (GeO₂), hydrogen silsesquioxane (HSQ), poly-silicon,silicon, etc. The non-metal oxide islands 130 may also be made ofprecious metals, including gold, silver, platinum, and palladium.

In one or more embodiments, a layer of non-metal oxide may be depositedon at least a portion of the metal oxide layer 120. A masking materialmay be deposited over the non-metal oxide layer and patterned to formmasked regions and exposed regions, as would be known in the art, andportions of non-metal oxide layer removed by etching to expose regionsof the underlying metal oxide layer surface 125.

In one or more embodiments, a masking material may be deposited over themetal oxide layer 120 and patterned to form masked region(s) and exposedregion(s) of the metal oxide layer surface 125, as would be known in theart. Non-metal oxide islands 130 may be deposited on the exposedregion(s) of the metal oxide layer 120, and the masking material removedto expose the underlying metal oxide layer surface 125.

In various embodiments, the non-metal oxide islands 130 may have athickness in the range of about 1 nm to about 100 nm, or in the range ofabout 5 nm to about 20 nm. The non-metal oxide islands 130 may havewidths in the range of about 5 nm to about 500 nm, or in the range ofabout 10 nm to about 100 nm.

In various embodiments, the non-metal oxide islands 130 form thesidewalls 142 of a trench 140, and the underlying metal oxide layersurface forms the bottom surface 147 of the trench 140, where the trench140 has a depth equal to the thickness of the non-metal oxide islands130.

One or more embodiments may include annealing the substrate, the layerof the metal oxide on the substrate, and the patterned non-metal oxidelayer on the metal oxide layer at a temperature in the range of about400° C. to about 600° C.

FIG. 4 is a cross-sectional view of a CNT-based device shown in FIG. 3with a bi-functional compound on the exposed portions of the underlyingmetal oxide layer surface in accordance with an exemplary embodiment.

In one or more embodiments, the CNT-based device 100 may be placed in asolution of a bi-functional compound 150, such that exposed portions ofthe underlying metal oxide layer surface may come in contact with thebi-functional compound 150 in solution. A functional moiety of thebi-functional compound 150 may bond with the exposed surface(s) of themetal oxide layer surface 125, while not bonding with the exposedsurfaces of the non-metal oxide islands 130. In various embodiments, ahydroxamic acid (—C(O)NR′OH), where R′ may be a hydrogen, moiety of thebi-functional compound 150 chemically bonds to the metal oxide formingthe bottom surface 147 of the trench 140.

In various embodiments, the CNT-based device 100 may be removed from thesolution of the bi-functional compound and rinsed to removebi-functional compound that has not been bound to the metal oxide layersurface 125. The bound bi-functional compound 155 may form a chemisorbedmonolayer along the bottom surface 147 of the trench(es) 140.

FIG. 5 is a cross-sectional view of a CNT-based device shown in FIG. 4with a functionalized bi-functional compound on the exposed portions ofthe underlying metal oxide layer surface in accordance with an exemplaryembodiment.

In one or more embodiments, the chemically bound bi-functional compound155 may be exposed to a solution of an alkyl nitrite, cycloalkylnitrite, or aryl nitrite (R—ONO), nitrosonium tetrafluoroborate (NO⁺:BF₄⁻), or aqueous sodium nitrite (NaNO₂) in the presence of an acid to forma functionalized bi-functional compound 158 on the bottom surface 147 ofthe trench(es) 140. In various embodiments, the alkyl nitrite may beselected from the group consisting of pentyl nitrite, 3-methylbutylnitrite, 2-methylbutyl nitrite, or combinations thereof.

In various embodiments, the nitrosonium tetrafluoroborate (NO⁺:BF₄ ⁻),aqueous sodium nitrite (NaNO₂) in the presence of an acid, alkylnitrite, cycloalkyl nitrite, or aryl nitrite (R—ONO) reacts with ananiline moiety of the bi-functional compound 155 to form a diazoniumsalt moiety 160. The diazonium salt moiety may reacted with a carbonnanotube to chemically bind the carbon nanotube to the metal oxidesurface region through the functionalized bi-functional compound 158.The functionalized bi-functional compound 158 anchors the carbonnanotubes 170 to the bottom surface 147 of the trench 140.

FIG. 6 is a cross-sectional view of a CNT-based device with carbonnanotubes (CNTs) bound to the functionalized bi-functional compound, asshown in FIG. 5 in accordance with an embodiment.

In one or more embodiments, the CNT-based device 100 with thefunctionalized bi-functional compound 158 may be exposed to a dispersionof carbon nanotubes 170 in an organic solvent. In various embodiments,the organic solvent may be toluene, tetrahydrofuran (THF), xylenes,dichloromethane (CH₂Cl₂), or chloroform (CHCl₃). The CNTs may besuspended in the organic solvent, where then may interact with thefunctionalized bi-functional compound 158 bound to the bottom surface147 of the trench(es) 140. The CNTs may chemically bond to the diazoniumsalt moiety 160 of the functionalized bi-functional compound 158, whichextends out into the dispersion of carbon nanotubes. The diazonium saltsmay form charge transfer complexes with the carbon nanotubes 170, whichmay then react to form covalent bonds with the CNT surface. The carbonnanotubes (CNTs) 175 are bound to the functionalized bi-functionalcompound 158.

In various embodiments, the carbon nanotubes have a length in the rangeof about 100 nm to about 2000 nm, or about 300 nm to about 600 nm.

In various embodiments, the CNTs may react with one or morefunctionalized bi-functional compound(s) 158. As the CNTs react with aplurality of functionalized bi-functional compound 158, the carbonnanotubes 170 may become aligned in a direction parallel to the longaxis of the trench 140. The CNTs may also align first along the longaxis of a trench 140 followed by reacting with the diazonium salt andformation of a covalent bond. Alignment of the carbon nanotubes mayprovide electronic overlap between neighboring CNTs in a trench to forma conductive path along the axis of the trench. Such alignment of thecarbon nanotubes may provide superior electrical connection yields thatare sufficient to build electrical devices on the surface of at leastthe CNT-filled trench.

In various embodiments, the CNT layer may have a thickness in the rangeof about 1 nm to about 10 nm, or about 1 nm to about 5 nm.

In various embodiments, trenches formed by the non-metal oxide islandsand the metal oxide layer may have a width in the range of about 10 nmto about 1000 nm, and a length in the range of about 300 nm to about2000 nm. In various embodiments, trenches formed by the non-metal oxideislands and the metal oxide layer may have a width in the range of about30 nm to about 500 nm, and a length in the range of about 500 nm toabout 1500 nm.

In a non-limiting example of an embodiment, a patterned substrate wasannealed for 30 minutes at 600° C. The annealed substrate was dippedinto a 5 mM toluene/ethanol (1:1) solution of arylamine hydroxamic acidfor 1 hour. The substrate was then rinsed with ethanol for 1 minute anddried The substrate was subsequently dipped into a sorted CNT-polymersolution for 1 hour. The substrate was subsequently dipped into an amylnitrite solution for 30 minutes. It was taken out and rinsed withtoluene for 1 minute and ethanol for 1 minute. The patterned substratewith a deposited CNT layer was dried by blowing nitrogen over thesurface followed by sonication in toluene for 5 minutes and ethanol for15 minutes. The CNTs have been selectively deposited onto hafnium oxidetrenches 150 nm wide.

Having described preferred embodiments of carbon nanotube based devicesand fabrication (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A carbon nanotube based device, comprising: asemiconductor substrate; an yttrium oxide (Y₂O₃) layer on thesemiconductor substrate; a plurality of germanium oxide (GeO₂) islandson the yttrium oxide (Y₂O₃) layer forming a trench having a long axisbetween the germanium oxide (GeO₂) islands; a monolayer of a pluralityof functionalized bifunctional compounds having a first reactive moietyand 4 second reactive moiety in the trench, wherein the first reactivemoiety is selectively chemically adsorbed onto exposed portions of theyttrium oxide (Y₂O₃) layer, and the second reactive moiety is adiazonium salt moiety; a plurality of carbon nanotubes, wherein thediazonium moiety of each of the plurality of functionalized bifunctionalcompounds is chemically bonded to one of the plurality of carbonnanotubes, and the plurality of carbon nanotubes are free of compoundsother than the functionalized bifunctional compound, and wherein thecarbon nanotubes are aligned in a direction parallel to the long axis ofthe trench, wherein the plurality of carbon nanotubes are selected fromthe group consisting of single-walled nanotubes (SWNTs), double-walledcarbon nanotubes (DWNTs), and multi-walled nanotubes (MWNTs), andwherein the carbon nanotubes have a length greater than a width of thetrench, so the bound carbon nano tubes are aligned with the long axis ofthe trench; and wherein the germanium oxide (GeO₂) islands have athickness in a range of about 1 nm to about 100 nm, and a thickness ofthe yttrium oxide (Y₂O₃) layer is also in a range of about 1 nm to about100 nm; wherein the trench formed by the germanium oxide (GeO₂) islandsand the yttrium oxide (Y₂O₃) layer has a width in a range of about 30 nmto about 500 nm, a length in a range of about 500 nm to about 1500 nmand a depth equal to the thickness of the germanium oxide (GeO₂)islands; wherein the carbon nanotubes have a thickness in a range of 1nm to about 5 nm and a length in a range of about 300 om to about 600nm.
 2. The carbon nanotube based device of claim 1, wherein the firstreactive moiety is a hydroxamic acid moiety that selectively binds tothe yttrium oxide (Y₂O₃) layer.